Overcoming Resistance to Endocrine Therapy in Breast Cancer: New Approaches to a Nagging Problem Med Princ Pract 2016;25(suppl 2):28-40 - DOI:10.1159/000444451 Fig. 1. Mechanisms of endocrine resistance. Various molecules are integrated in a complex network, each playing a role in the development of hormonal therapy resistance in breast cancer. RTK, with their down-stream signalling molecules (e.g. FAK, Src, Akt and PI3K) promote membrane oestrogen-independent ER phosphorylation, leading to constitutive activation, to induce a more invasive and malignant phenotype. Loss/reduction of ERβ can also lead to increased ERα-promoted activation. Other cell-signalling receptors such as integrins contribute through synergizing of the action of RTK and the activation of FAK. Environmental stress leads to an elevated expression of Hsp and p38, nboth of which enhance resistance by maintaining high levels of RTK and over-expression of ER co-activator molecules which stimulate ER signalling. Endocrine resistance can also be due to loss/mutation of ER by promotion of EMT through mediators such as SNAIL. ERα can also be downregulated by specific miRNA. Tamoxifen metabolism to its more active metabolite endoxifen can be reduced by a lower activity of CYP2D6 enzyme variants, as well as by an increased efflux through an upregulated P-glycoprotein pump. TAM = Tamoxifen; ENDOX = endoxifen; Co-A = co-activator. © 2016 S. Karger AG, Basel - CC BY-NC 3.0

Overcoming Resistance to Endocrine Therapy in Breast Cancer: New Approaches to a Nagging Problem Med Princ Pract 2016;25(suppl 2):28-40 - DOI:10.1159/000444451 Fig. 2. EMT is determined by various molecular events; examples are illustrated. Most commonly, several groups of transcription factors can act to downregulate epithelial markers such as E-cadherin. Regulation of such transcription factors by molecules such as YB-1 can sustain EMT. Conversely, miRNAs can block EMT by targeting its mediators. © 2016 S. Karger AG, Basel - CC BY-NC 3.0

Overcoming Resistance to Endocrine Therapy in Breast Cancer: New Approaches to a Nagging Problem Med Princ Pract 2016;25(suppl 2):28-40 - DOI:10.1159/000444451 Fig. 3. Integrated miRNA network. miRNAs can be involved in a complex network where they can target the same or different mRNAs affecting different processes. miRNA-221/222 can target and suppress the expression of phosphatase and tensin homologue and p27kip1, thus alleviating the repression of MAPK and HER2, respectively, ultimately leading to a high cell proliferation. miRNA-221/222 also directly targets ER, pushing cancer cells toward EMT and endocrine resistance. On the other hand, miRNA-124 and miRNA-145 attenuate EMT and resistance to hormonal therapy by targeting EMT-related transcription factors (SLUG and SNAII), thus preventing their inhibition of E-cadherin. miRNA-124 also targets and degrades ETS, which leads to a lower cell proliferation. miRNA-145 represses the migration capacities of cancer cells by targeting JAM-1 and fascin. miRNA-9 plays a crucial role in the induction of EMT through direct repression of E-cadherin and indirect upregulation of vimentin. © 2016 S. Karger AG, Basel - CC BY-NC 3.0

Overcoming Resistance to Endocrine Therapy in Breast Cancer: New Approaches to a Nagging Problem Med Princ Pract 2016;25(suppl 2):28-40 - DOI:10.1159/000444451 Fig. 4. miRNA manipulation as a treatment modality. miRNA binds to a specific target mRNA to facilitate the downregulation of target proteins. miRNA mimics are synthetic oligonucleotide duplexes that have the same sequence as the endogenous miRNA and function as replacements for the downregulated miRNAs (acting as tumour suppressors). Antagomirs (also called antimiRs) are oligonucleotides that have complementarity with the target miRNA, thereby binding and preventing them from interacting with the target mRNA, allowing a normal translation. An miR mask is complementary to a sequence in the mRNA and binds to it without affecting the mRNA integrity or translation, yet it prevents the endogenous miRNA from binding. A sponge miR mask differs from the miR mask in that it binds to any mRNA with a similar target sequence and is therefore miRNA seed specific and not gene specific. © 2016 S. Karger AG, Basel - CC BY-NC 3.0